This invention relates to a process for preparing potassium tripolyphosphate (KTPP) from sodium tripolyphosphate (STPP) which produces no waste discharge stream. In particular, it relates to the use of an ion exchange resin loaded with potassium or hydrogen ions through which sodium tripolyphosphate is passed, followed by regeneration of the sodium tripolyphosphate from the exhausted ion exchange resin.
Potassium tripolyphosphate, K.sub.5 P.sub.3 O.sub.10, is a low volume specialty condensed phosphate which finds utility in phosphate-built liquid detergents, in TiO.sub.2 processing as a latex paint dispersant, and in some specialty food processing applications. It is believed that at the present time there is only a single United States manufacturer of potassium tripolyphosphate and that the process that manufacturer uses follows the traditional steps for polyphosphate synthesis. That is, an orthophosphate solution is formed by the reaction of aqueous phosphoric acid with the appropriate alkali metal salt until a ratio of 5 alkali metal ions to 3 orthophosphate ions is contained in the solution, followed by drying and calcination to drive off water of hydration and produce the condensed alkali tripolyphosphate. For example, an alkali metal tripolyphosphate would be prepared according to the following equation: EQU 5MOH+3H.sub.3 PO.sub.4 .fwdarw.M.sub.5 P.sub.3 O.sub.10 +7H.sub.2 O
where M is usually Na or K.
Although the chemistry of this process is straightforward, there are several problems with the manufacture of potassium tripolyphosphate which do not occur in the manufacture of sodium tripolyphosphate. For example, during calcination the partially converted potassium phosphate species tend to lump and cake, resulting in incomplete dehydration, adherence to the calciner surfaces (which requires increased maintenance), inefficient heat utilization, an undesirable particle size distribution (which requires extensive further milling and screening), and the formation of highly insoluble potassium metaphosphates, which are very undesirable in aqueous applications requiring clear liquids. In addition, potassium tripolyphosphate cannot readily be made in sodium tripolyphosphate manufacturing equipment without incurring quality problems due to cross contamination and operability problems due to plant turndown limitations for practical operation.